Once the trees were removed, the workers ground the stumps, but not very deep. The owner said the ground up stuff was too acidic to grow anything. What can we mix over the area to reduce the acid and will grass or some type of plants grow then?

I have never heard of palms being acidic before. Was he trying to sell you something? If you intend to plant grass, I would remove as much of the 'grindings' as possible and replace with whatever soil is common in your area. The grass isn't going to grow in what is essentially mulch.

If you are really concerned, get some litmus paper and do a litmus soil test.

Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and proclaiming...."WOW What a Ride!!" -Mark Frost